J. Org. Chem. 2001, 66, 1775-1780
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Photo-Fries Rearrangements of Phenyl Phenylacylates in Polyethylene Films. Comparison of Reactivity and Selectivity with 1-Naphthyl Phenylacylates Weiqiang Gu† and Richard G. Weiss* Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227
[email protected] Received October 9, 2000
The fates and kinetics of recombination of singlet radical pairs generated by photolyses of three phenyl phenylacylates have been examined in unstretched and stretched polyethylene films. Comparisons with results from photolyses of analogous 1-naphthyl phenylacylates in the same media lead to the conclusions that (1) phenoxy is less reactive overall than 1-naphthoxy toward a common phenylacyl radical but (2) the constrained cages in which the radical pairs reside exert greater control over the movements of the 1-naphthoxy/phenylacyl pairs. The reasons for these observations are discussed in the context of the shapes and van der Waals volumes of the radical pairs, the void volumes of sites in native polyethylene films, and the electronic properties of the aryloxy radicals. Introduction Investigations of the factors influencing the selectivity and kinetics1 of radical-pair recombination processes in cages provided by polymeric media have led to several interesting discoveries, as well as many questions. Our approach to answering these questions has involved studies of the photo-Fries rearrangements of 1- and 2-naphthyl esters2-4 and photo-Claisen rearrangements of aryl benzyl ethers5 in polyethylene (PE) and other polyolefinic media.3,6 The difference between the size and shape of the intermediate phenylacyl radicals (from lysis of the esters) and benzylic radicals (from lysis of the ethers or decarbonylation of the initially formed phenylacyl moieties upon irradiation of the esters) has been found to influence the selectivity and kinetics of their addition to aryloxy partners in the confines of PE cages.5 In addition, the large van der Waals volumes of the reactant esters and ethers (with respect to the mean hole free volumes of the polyethylene media) and slow rates of relaxation of reaction cavity walls (with respect to the rates of in-cage radical-pair recombinations) suggest that product formation is mediated by “templating” effects, also.3 Despite these efforts, several important mechanistic questions remain unanswered. Among them is, “Are rates and modes of recombinations dictated principally by the manner in which a singlet radical-pair interacts with its cage or with itself?” Here, we address this question by comparing the photo-Fries reactions of some phenyl † Current address: Department of Chemistry, University of California, Berkeley, California 94720-1460. (1) Gu, W.; Weiss, R. G. Tetrahedron 2000, 56, 6913. (2) Cui, C.; Weiss, R. G. J. Am. Chem. Soc. 1993, 115, 9820. (3) Gu, W.; Hill, A. J.; Wang, X.; Cui, C.; Weiss, R. G. Macromolecules 2000, 33, 7801. (4) Gu, W.;Warrier, M.; Ramamurthy, V.; Weiss, R. G. J. Am. Chem. Soc. 1999, 121, 9467. (5) Gu, W.; Warrier, M.; Schoon, B.; Ramamurthy, V.; Weiss, R. G. Langmuir 2000, 16, 6977. (6) Zimerman, O. E.; Cui, C.; Wang, X.; Atvars, T. D.; Weiss, R. G. Polymer 1998, 39, 1177.
phenylacylates (1) and 5,6,7,8-tetrahydro-1-naphthyl phenylacetate (2) with those from analogous 1-naphthyl phenylacylates in PE films (eq 1).1 The van der Waals
size and volume of the highly substituted phenyl ester 2 are near those of the corresponding 1-naphthyl ester, 1-naphthyl phenylacetate (3a). Our previously developed radical-clock method for measuring the rates of in-cage radical recombinations1 has been employed in the analyses. From these results, it is possible to determine whether the size (and shape) or reactivity of an aryloxy radical has the greater influence on the kinetics of photoproduct formation during recombination with its acyl partner in a constrained environment.
Results General Description of Reaction Pathways of 1 and 2. Photo-Fries rearrangements of phenyl esters
10.1021/jo001455+ CCC: $20.00 © 2001 American Chemical Society Published on Web 02/14/2001
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J. Org. Chem., Vol. 66, No. 5, 2001
Gu and Weiss Scheme 1
Table 1. Relative Photoproduct Yields (%) from Irradiations of 2 mM 1 and 2 to e 20% Conversions in N2-Saturated Solutions at 22 °C ester
solvent
2-AP
4-AP
2-BP
4-BP
BnOP
BnP
POL
Sol-Bnd
(Bn)2
1aa
hexane cyclohexane hexane hexane cyclohexane
35.2 ( 0.6 41.9 ( 1.1 37.2 ( 0.2 42.9 ( 0.5 55.2 ( 0.4
12.8 ( 1.2 11.7 ( 0.5 22.0 ( 1.8 c c
1.6 ( 0.1 1.2 ( 0.1 0.8 ( 0.3 7.1 ( 0.1 5.2 ( 1.7
1.0 ( 0.1 0.4 ( 0.1 1.3 ( 0.4 c c
6.0 ( 0.2 5.0 ( 0.2 10.6 ( 0.5 5.4 ( 0.1 4.0 ( 0.3
0.8 ( 0.1 1.7 ( 0.1 0.6 ( 0.2 3.0 ( 0.1 4.1 ( 0.8
19.8 ( 0.2 17.3 ( 0.9 10.1 ( 1.4 21.5 ( 0.4 14.8 ( 1.0
2.7 ( 0.3 2.6 ( 0.1 2.0 ( 0.4 3.9 ( 0.1 3.4 ( 1.5
14.3 ( 0.4 10.9 ( 0.4 15.4 ( 0.5 7.9 ( 0.1 5.9 ( 0.5
1b 2b
a There were 3 unidentified peaks in each solvent, accounting for 5.8% (hexane) and 7.2% (cyclohexane) of the total product yields. The yields for the two unidentified products: u-1, 3.9% in hexane and 3.2% in cyclohexane. u-2, 4.4% in hexane and 4.2% in cyclohexane. c
